1. Field of the Invention
This invention relates to a reaction chamber and method of silylation in conjunction with the fabrication of semiconductor devices.
2. Brief Description of the Prior Art
In the planar fabrication of semiconductor devices on a semiconductor wafer, a necessary step in the fabrication procedure requires printing of a pattern layer on the wafer surface, this printing step generally being repeated several times during the fabrication procedure. The patterns are generally printed on a resist layer. Resist layers are generally spun onto the surface of the semiconductor wafer under fabrication and are then exposed to light with either the exposed or unexposed portion of the resist then being removed and fabrication operations then taking place in the region from which the resist has been removed.
In order to fabricate such semiconductor devices in the minimum possible area and with accuracy, it is necessary that such patterns be printed on the semiconductor wafer under fabrication using the smallest possible pattern dimensions (i.e., high resolution) and that the patterns from layer to layer be placed within a predetermined maximum spatial tolerance of each other. This spatial tolerance is referred to as alignment.
The resolution of optical microlithography is being extended through the continued refinement of the photoresist process and exposure tools. Other evolutionary processes, such as surface-imaging, are also beginning to play a role in extending the practical resolution limit of optical microlithography.
One such surface imaging process is known as the DESIRE process. The DESIRE process, which is a surface imaging process where the features are developed in an anisotropic oxygen plasma, can extend the practical resolution limit of optical microlithography. The DESIRE process differs from conventional lithography mainly in two steps, these being (1) silylation and (2) dry development. The DESIRE process is set forth in an article by B. Coopmans and B. Roland entitled "DESIRE: a novel dry-developed resist system", SPIE, Vol. 631, an article by Cesar Garza et al. entitled "Mechanism of the DESIRE Process", SPIE, Vol. 771, Advances in Resist Technology IV (1987), pages 69 to 76, an article by Cesar Garza et al. entitled "Preliminary Performance Characterization of the DESIRE Process", SPIE, Vol. 920, Advances in Resist Technology and Processing V (1988), pages 233-240 and an article entitled "Manufacturability Issues of the DESIRE Process" SPIE, Vol. 1086, Advances in Resist Technology and Processing VI (1989), pages 229 to 237, the contents of each of which are incorporated herein by reference.
In the DESIRE process, silicon is selectively incorporated from the gas phase into the exposed areas of the photoresist, the photoresist being a standard prior art photoresist, such as, for example, diazoquinone and a dye wherein the exposed regions are converted into an acid, after coating and exposure of the photoresist. The dye prevents the light impinginq upon the
n photoresist from travelling entirely therethrough and limits such exposure to only a small depth at the surface of the photoresist. This provides several advantages over the art prior to the DESIRE process, these including the fact that (1) the side walls of the exposed region of the photoresist tend to move away from the vertical with increased depth and affects pattern quality and accuracy and (2) light reflected from neighboring substrates features can have an adverse impact in the dimensions and quality of the resist features, whereas it will not be a problem in the case of the DESIRE process. Accordingly, operation on the photoresist in the DESIRE process must now be concerned only with the surface regions thereof. The selective silylation results in a dramatic decrease in etch rate of the exposed areas in an anisotropic oxygen plasma. In the DESIRE process flow, one single layer of PLASMASK resist, which is a novolac resin having phenolic groups which are available for reaction plus a dy and a sensitizer is spincoated onto the substrate being fabricated and prebaked to a self-planarizing layer. Typical resist thickness ranges from 1.5 to 2.5 microns depending upon the substrate. Prebake is carried out on a hot plate at temperatures of 90 to 110 degrees C. for 30 to 60 seconds, depending upon the resist thickness. After patternwise exposure on standard exposure equipment, the wafers are treated with a vaporized silylating agent (e.g. HMDS), such silylating agents being well known in the art This silylation is carried out at elevated temperature in the 150.degree. to 180.degree. range with a preferred temperature of 160.degree.. As a result of the photochemical modifications of the resist during exposure, the exposed areas are selectively silylated in such a way that silicon is incorporated into the top 100 to 250 nanometers of these exposed parts. The incorporated silicon is chemically bound to the resin so that the silylated wafers remain stable for an extended period after silylation. The wafers are then developed in an oxygen plasma. During this plasma treatment, the silicon is converted into silicon dioxide which forms a thin protective mask that stops etching in the exposed areas. The unexposed parts do not contain silicon and are removed during the development or etching step. Extremely vertical resist profiles are obtained when an anisotropic plasma is used for the dry development.
In the silylation step, a suitable silicon containing chemical, such as hexamethyldizilazane (HMDS) reacts with the available hydroxyl or phenyl groups of the novolak resin component of the photoresist.
The most common procedure for carrying out silylation is to bubble an inert gas, such as nitrogen, through a vessel containing HMDS or other suitable silylating agent, to a reaction chamber. In such a reaction chamber, the wafer is located on a hot plate at a temperature suitable for this process.
The silylation process includes (1) a pre-silylation bake to increase the silylation contrast by thermally cross-linking the photoresist and (2) the silylation step itself, where the photoresist reacts with the silylating agent from the gas phase. An optimized process typically consists of a pre-silylation (also known as a post-exposure bake) at about 160 degrees C. for about two minutes followed by a silylation period of about four minutes at the same temperature as the post-exposure bake. This implies that the typical processing time is six minutes plus overhead, which is the time required to transport the wafer in and out of the chamber, plus the time required for the equipment circuitry to activate the proper set of valves or a throughput of up to about ten wafers per hour with present day equipment. It is currently the belief in the art that it is a requirement that the silylation bake be performed under the exact temperature conditions as the silylation step to control the process in a manufacturing environment.
It has been determined that in prior art reaction chambers utilized in conjunction with the DESIRE process, particularly in the near half micron photolithography area, dimensions are sensitive to changes in flow rate of the silylating agent across the wafer. In presently available equipment, such as Plasmaster-Si marketed by JSR, a consistent improvement in the line-width uniformity has been noted by changing the pattern and diameter o holes that allow the diffusion of the silylating agent through the top plate and into the reaction chamber. A porous ceramic material has been used as the top plate which has provided another slight improvement in uniformity. However, a systematic reduction in the line-width caused by weaker silylation still exists in the center of the wafer. It is therefore apparent that a better system is required to improve uniformity of flow of the silylating agent within the reaction chamber, especially in conjunction with photolithographic lines of about 0.6 microns and below.